The present invention relates to a wet gas purification method for removing ammonia from a gas. More particularly, the present invention relates to a wet gas purification method and a wet gas purification system suitable for removing ammonia from a gas such as a coal or heavy oil gasification gas.
Conventionally, in a wet purification system for a coal or heavy oil gasification gas, sulfuric acid is added upstream of a hydrogen sulfide (H2S) absorption tower as a pH adjuster. This allows for the removal of ammonia (NH3), which is a source of fuel NOx in gas turbines, during the water-washing step for removing impurities in the gas.
Specifically, if a gas containing ammonia is burned in a gas turbine without being purified, the gas becomes a NOx source, so that it is necessary to recover as much ammonia as possible. To absorb the ammonia, a method for decreasing the pH of an absorbent in a water-washing tower is carried out, by which most of the ammonia can be removed. As a pH adjuster, sulfuric acid is preferably used in adjusting alkali of ammonia. The pH of the water-washing tower can be reduced by the addition of the sulfuric acid.
The ammonia removed in the above-described water-washing step is taken out by an ammonia stripper. Since sulfuric acid is added to the absorbent, ammonium sulfate exists in the discharged solution from the ammonia stripper. The ammonia is recovered as aqueous ammonium sulfate.2NH3+H2SO4→(NH4)2SO4 
To expel ammonia components from the aqueous ammonium sulfate and to recover ammonia as aqueous ammonia by a stripping method, caustic soda (NaOH) is added before the stripping. Usually, the caustic soda is charged into a neutralization tank provided upstream of the stripper and is mixed. Then, the mixed liquid is sent to the stripper. The addition of NaOH yields sulfuric acid and ammonia again as described in the following formula. Then ammonia is recovered.(NH4)2SO4→2NH3+H2SO4 
On the other hand, in the above-described conventional system, one of major reasons for adding sulfuric acid is to facilitate the operation control of the system. That is to say, if the pH value of the liquid is decreased by sulfuric acid, ammonia can be recovered and removed. There is no need for monitoring and controlling the amount of ammonia at the following step of the water-washing step, and the control of pH suffices for operation. Because of such ease of operation, a method in which sulfuric acid is added has been used.
Also, by decreasing the pH value, hydrogen sulfide is not removed in the water-washing step, but passes through easily. A higher pH value presents a disadvantage of dissolution of hydrogen sulfide in the liquid, making the treatment of effluents complicated. Specifically, the conventional practice involves changing the pH value between the separation/removal step of ammonia and the removal step of hydrogen sulfide so that in the ammonia separation step, hydrogen sulfide does not dissolve in the liquid, and is not removed from the gas.
Moreover, the conventional practice requires the addition of sulfuric acid and caustic soda, which are costly. For example, to treat 1000 ppm of ammonia existing in the gas, ½ mole of sulfuric acid is needed for every 1 mole of ammonia, which increases the chemical cost. Also, in order to treat the nitrogen components in the effluent from a water-washing tower, which contains absorbed ammonia, at least 2 moles of caustic soda (NaOH) are required per 1 mole of sulfuric acid. Further, an amount of caustic soda equivalent to the amount of ammonia is needed. Therefore, the chemical expense is great. Thus, while the ammonia can be recovered, the after-treatment steps are complicated and increase in number.
Further, since the recovered aqueous ammonia accounts for about 20 percent by weight, there is no other choice but to treat it as waste, which poses a problem of treatment cost.